Legislation restricting the amount of pollutants that may be emitted into the atmosphere is becoming increasingly strict. One category of pollutant that is legislated against by inter-governmental organisations throughout the world is unburned hydrocarbons (HCs). Unburned HCs of varying composition are typically present in exhaust gases produced by various types of mobile or stationary engine, such as spark ignition engines, compression ignition engines and combustion turbines.
Saturated hydrocarbons having a relatively short-chain, such as methane (CH4), ethane (C2H6) and propane (C3H8), are present in the exhaust gas produced by an internal combustion engine. These short-chain, saturated hydrocarbons are not very reactive compared to other species of hydrocarbon that make up the hydrocarbon component of an exhaust gas and they are particularly difficult to catalytically oxidise over a catalytic convertor in exhaust gases having a “lean” gas composition. Methane is also a potent greenhouse gas (GHG).
To compensate for the low reactivity of short-chain, saturated hydrocarbons, high temperatures and high loadings of Pt and/or Pd are often used to oxidise them. Platinum and palladium are precious metals. It is expensive to use a high loading of Pt and/or Pd in a catalytic convertor.
There are also internal combustion engines that are purpose-built or have been modified to use natural gas (NG) as a fuel. Engines are known that can use NG as a fuel in a variety of ways, such as alone (e.g. a “dedicated” NG engine) or in conjunction with another fuel (e.g. a bi-fuel engine) where the engine may be run on one fuel at a time or both fuels may be used in unison. When NG is used as a fuel, the exhaust gas produced by the engine contains significant quantities of methane (so-called “methane slip”).
Natural gas (NG) typically comprises a hydrocarbon (HC) gas mixture and small amounts of carbon dioxide (CO2), hydrogen sulfide (H2S), water vapour (H2O) and nitrogen (N2). The main component of NG is methane (CH4), but relatively small amounts of ethane (C2H6), propane (C3H8) and other hydrocarbons are usually also present. There is interest in using natural gas (NG) as a fuel for engines, particularly in the form of either compressed natural gas (CNG) or liquefied natural gas (LNG). For vehicular applications, the use of CNG as a fuel is typically preferred over LNG because CNG generally has both lower production and storage costs compared to LNG.